(1) Field of the Invention
This invention relates to a method of manufacturing a structure immune against stress corrosion cracking. More particularly, it relates to a novel method of repair-welding austenite type stainless steel pipes which is appropriate for preventing the stress corrosion cracking that occurs in the welding heat-affected zones of the steel pipes of a chemical plant, an atomic energy plant etc. in any corrosive atmosphere.
(2) Description of the Prior Art
In general, when austenite type stainless steel is heated to 500.degree.-800.degree. C., carbides precipitate in the grain boundary. The precipitation of carbides is noted even when the material is heated to a high temperature for a short time as in welding. Under specific corrosive environments, stress corrosion cracking is liable to occur due to a residual tensile stress or external tensile stress acting on such a part.
Also, when ferrite type stainless steel is subjected to welding, a residual welding stress of tension appears in the welded part, and stress corrosion cracking often occurs under corrosive environments. Also in case of low alloy steel and high tension steel, joints subjected to ordinary weldings are prone to undergo stress corrosion cracking in the atmosphere of hydrogen sulfide.
In this manner, the stress corrosion cracking usually takes place by the combination of the material factor and the corrosive environment. As expedients for preventing the stress corrosion cracking, there have been taken a countermeasure for the austenite type stainless steel in which the precipitation of carbides is prevented and a countermeasure for the ferrite type stainless steel in which the material is made difficult to corrode under the corrosive environments.
Since heat is applied in ordinary welded joints, the weld zone and the vicinity thereof reach a considerably high temperature and simultaneously undergo a high residual tensile stress, which provides a cause for a tendency to undergo stress corrosion cracking. It is therefore necessary to prevent the rise of the temperature of a part which is exposed to corrosive environments. If the high residual tensile stress can be lowered, the stress corrosion cracking-resistance can be made conspicuously high.
As processes for reducing the residual tensile stress, there have heretofore been the stress relief annealing, the Linde process, the overstraining, the peening, the vibrational stress relieving, etc. The Linde process is such that, after completion of welding, both the sides of a weld zone are heated. The overstraining is such that, after welding, an external force is exerted to give rise to a plastic strain locally. Although these processes can moderate the residual tensile stress to some extent, neither of them can add a residual compressive stress. The peening involves a difficulty in point of reproducibility.
In this manner, any of the conventional processes for reducing the residual tensile stress cannot bring forth a satisfactory result.
As an expedient for preventing the stress corrosion cracking of the weld zone of an austenite stainless steel pipe which is used under any corrosive environment, there has been a method of welding wherein after a preceding step of welding to the extent that a cooling medium does not flow out to the exterior, a succeeding step of welding is carried out while forcibly cooling the inner surface of the steel pipe. It has been known that a joint welded by this method is more excellent in the stress corrosion cracking-resisting property of the inner surface of the pipe than a joint fabricated without forcibly cooling the inner surface of the pipe during welding. The inventors, however, have revealed as the result of various studies that when a defect found out by an inspection in the course of or after completion of the welding in this welding process is locally welded for repair, a residual tensile stress appears in the weld zone in dependence on conditions of the repair welding. Accordingly, as regards the welded joint which poses the problem of the stress corrosion cracking and which includes at a part thereof the defect caused during the welding, the mere welding for repair of the defect part is unsatisfactory in that there is a high possibility of causing cracking again.
Further, a repair welding process for a steel pipe is disclosed in U.S. Pat. No. 4,049,186. This process is such that, while letting water flow in the steel pipe, the build-up welding is done in a weld zone on the outer periphery of the steel pipe. It is described that, according to the process, the tendency for stress corrosion cracking that is usually caused in the inner surface of the steel pipe by a residual tensile stress attributed to welding when the inner surface of the pipe is exposed to a corrosive atmosphere as in an atomic energy plant is reduced.
However, in case where the wall of the steel pipe is thick, this repair welding process cannot make the residual tensile stress of the inner surface of the steel pipe ascribable to the repair welding sufficiently low and cannot satisfactorily prevent the stress corrosion cracking. That is, in the case of the thick-walled steel pipe, even when the temperature difference between the build-up welding zone and the inner surface of the steel pipe is great, obviously a local temperature difference or temperature gradient in the inner surface of the steel pipe is small, so that the residual tensile stress cannot be sufficiently diminished in the inner surface of the steel pipe. The reason why the build-up welding on the outer surface of the steel pipe will be insufficient is that the outer surface of the steel pipe is expanded by being subjected to the shrinkage deformation of a deposit metal and the heating of the build-up welding, the expanded part being constrained at both it ends to undergo a compressive deformation, which gives a shrinkage deformation attendant upon cooling after the welding. However, the shrinkage deformation of the outer surface of the steel pipe owing to the build-up welding is attained only by a thermal expansion based on heating attendant upon the fusion of the outermost surface of the steel pipe and the welding heat, and the effect of reducing the residual tensile stress of the inner surface of the steel pipe is not sufficiently demonstrated.